In wind-power generation, rotational energy is converted into electrical energy by transmitting the rotary action of propellers to a rotor shaft of a power generator. The output of a power generator is either connected to system power, or stored in batteries and used as electrical power when necessary. Since the generating electrical power depends on the specifications of the power generator, the power generator is selected to match the electrical power generating facility.
There are several types of power generators, including, for example, a type that uses coils, and a type that uses permanent magnets as field magnets, or, regarding their construction, a radial type, in which a stator surrounds the outside of a cylindrical rotor, and an axial type, in which a stator faces a disc-shaped rotor in the axial direction, etc. Each type has its own advantages and disadvantages, but the permanent magnet type is used as field magnets if the efficiency of power generation is significant because it can generate a stronger magnetic field, increase magnetic flux linked to the armature coil and raise the induced voltage compared to the coil field magnet type having the same physical size.
Since electrical power energy depends largely on a power generator's efficiency, there is a demand for power generators having higher efficiency. In particular, in the wind power generation, generators are used at a rotational speed of no more than several hundred rpm, which is considerably lower than in other forms of electrical power generation. Since the generated voltage is proportional to a power generator's rotational speed, if a power generator employed in the other power generation method is used as a wind power generator without additional modifications, the generated voltage will drop. Charge pump circuitry is thus essential in order to raise the generated voltage, resulting in a cost increase. The voltage can be raised by raising the rotational speed. While the voltage may also be raised by interposing an acceleration gear between the propeller shaft and the rotor shaft of the power generator, this method is undesirable because the gear may cause gear-induced torque losses and may become a factor of noise generation and reduced equipment reliability along with an important cost increase factor.
In addition, the generated voltage is proportional to the number of magnetic poles of the field magnets. While the number of coils or magnets of a rotor can be increased in order to increase the number of magnetic poles, magnetic field of the field magnets may decrease because each coil or magnet must be small. Accordingly, one option may be to enlarge the diameter of a rotor for increasing magnetic poles by increasing the number of coils or magnets of the same size keeping the magnetic field of the field magnets. Such method is practical, however, enlarging the diameter of the rotors of a power generator means making the overall diameter of the power generator larger, which is undesirable for the following reasons in case of wind power generation.
While there are two types of wind turbines, i.e. a vertical axis type, in which the axis of rotation is perpendicular to the direction of the wind, and a horizontal axis type, in which it is parallel thereto, the horizontal axis type is generally used due to its high efficiency at high wind speeds. In the horizontal axis type, the size of the power generator directly connected to the rotor shaft at the center of the propeller affects the wind-capturing capacity of the propeller. Namely, as is shown by the wind power generator of FIG. 11, the wind-capturing area of the propeller is decreased and the rotational force of the propeller is reduced in proportion to the enlargement of the nacelle 61 housing the power generator 62. In other words, the efficiency of wind power generation drops as the power generator becomes large. JP2002-153036A discloses a coreless power generator applied to wind power generation. Such generator significantly reduces the wind-capturing area if the outside diameter of the power generator becomes larger.